The invention relates to a process for surface treatment of opening rollers for open end spinning.
Open end spinning is at present the most economical way of producing yarn from short fiber. The most essential components of an open end spinning unit are the opening roller and the spinning rotor. The opening roller separates the feed sliver into its individual fibers, just a few micrometers thick, removes impurities and feeds the fibers through a feed tube into the spinning rotor, where they are reassembled to form a yarn. The working of the opening roller has a crucial bearing on the stability of the spinning process and on the quality of the yarn product.
A common form of opening roller is a ring-shaped structure made of aluminum or steel, whose circumferential surface is equipped with a spiral-shaped slot fitted with a finely toothed steel tape - the wire clothing- fixed in place by caulking. FIG. 1 shows a partly broken-away opening roller ring comprising the aluminum body 1 and the clothing wire spiral 2.
Examples of opening rollers and toothed tapes or wire may be found inter alia in U.S. Pat. Nos. 2,937,413, 4,233,711, 2,731,676, 4,435,953 and 3,833,968. The toothed tape clothing are usually produced by rolling an initially round wire into the characteristic cross-sectional shape and then stamping out the teeth from the flat part of this profile tape. Such a clothing wire is shown in cross section in FIG. 2a and in a partial side view of FIG. 2b. Sometimes the tooth flanks are subjected to a mechanical after-treatment by grinding. This is described for example in U.S. Pat. No. 4,233,711.
The clothing wire is at this stage still in the raw state. The edges of the teeth of the raw wire are sharp and in part very rough. Opening rollers equipped with a clothing wire in this state have completely unacceptable spinning characteristics; the fine fibers are destroyed or become lodged in the rough areas of the teeth only to become detached from time to time and create thick places in the yarn product.
It is therefore common practice to subject clothing wires for opening rollers, prior to mounting on the roller body, to an electrolytic or chemical treatment. This treatment serves to round the sharp edges and generally improves the surface quality. To this end, the raw wire is successively degreased, descaled, pickled and deburred in various electrolytic and/or chemical baths. Thorough rinsing is necessary between the actual operations, and this results in the entire treatment being laborious and costly.
The surface state of the teeth resulting from this treatment is known as needle finish. It is considered absolutely mandatory for satisfactory working of an opening roller equipped with wire clothing. A reference to this needle finish may be found for example in U.S. Pat. No. 5,006,367, column 2, lines 9-10.
It is also common practice to protect the teeth of opening rollers from wear and hence to prolong the useful life of opening rollers by specific surface-technological measures. A particularly effective measure is the application to the needle finished, wire clothed opening roller of a dispersion coat consisting of autocatalytically deposited nickel with embedded diamond particles. This is described inter alia in Metalloberflache 1984, No. 4, page 139, or Textile Month, May 1981. Opening rollers equipped with such a nickel-diamond coating have service lives which exceed those of uncoated ones by a factor of from five to ten.
Like the above-described deburring and rounding treatment of raw wire, a nickel-diamond coating requires multi-stage treatment in dip baths, so that it is desirable to combine the two processes in an economical manner. Such a combination would have appreciable advantages:
a) Manufacturing opening rollers using the significantly less costly, non-deburred raw wire represents an appreciable cost saving. The actual deburring is merely an additional pretreatment step prior to the nickel-diamond coating which is carried out in any case and therefore represents only an insignificant additional cost. PA1 b) Owing to the geometrically exact position of the wire on the roller body, the deburring process is more defined and more reproducible than in the hitherto customary bundle or in a continuous process, reducing the proportion of rejects due to surface flaws. PA1 a) introducing the opening roller into a sealing bath in such a way that this sealing bath fills even the smallest voids between the raw wire and the basic body of the roller, PA1 b) rinsing off the opening roller clean on the outside, PA1 c) heat treating the opening roller, and PA1 d) subjecting the opening roller thus pretreated to deburring and antiwear coating in a conventional manner.
Prior endeavors in the art have indeed confirmed the basic feasibility of such a combined process. However, it has hitherto not been possible to mass produce a reliable product. This is because of a peculiarity of the manufacture of wire clothed opening rollers which leads to damage following a very long latent period:
To be able to pull the clothing wire into the spiral-shaped slot of the roller body, the slot has to be somewhat wider than the wire foot. In addition, variations in the rolling of the wire and in the wear of the tools for cutting the slots are responsible for size differences which lead to voids of variable size between the wire and the slot wall of the roller body. It has been found to be technically impossible in a mass production process to eliminate or seal off these voids using the caulking operation carried out for fixing the wire on the body of the roller.
If a wire clothed opening roller is dipped into a deburring bath, the aggressive fluid of the bath will also penetrate into the above mentioned voids and attack the metal surfaces. Initially this is no problem and is in general hidden by the subsequently applied nickel-diamond coating. Since, to achieve maximum wear resistance, the coating is followed by a heat treatment at from 250.degree. to 350.degree. C., the fluid remaining in the voids will also evaporate completely, leaving behind dry salts. The opening rollers subsequently deburred and coated in a single operation do indeed appear to be free of flaws directly following the surface treatment.
However, if such rollers come into contact with higher atmospheric humidity over a period, the dry salts will regain their chemical activity and restart the interrupted corrosion processes. In spinning mills, where opening rollers are used in accordance with their intended use, the humidity is in fact artificially raised to avoid electrostatic charge buildups so that sooner or later, a large proportion of the rollers will fall victim to corrosion on an unacceptable scale.
Acceptable to an end user of opening rollers means a maximum proportion of &lt;10% of opening rollers with individual rust spots.
It is known of aluminum alloys that they are attacked not only by alkaline but also by acidic media and that, once started, corrosion processes are in practice impossible to stop. Opening rollers based on bodies made from such alloys will eventually show corrosion efflorescence which causes even firmly adhering and stable surface layers to spall.
Roller bodies made of iron materials are altogether prone to rusting, so that an opening roller made entirely of steel will usually require an all-over corrosion protection. The subsequent formation of rust by the mechanism described above leads to similar damage as produced by the corrosion of aluminum and is therefore similarly unacceptable.
There have been attempts to fill out the unavoidable void between the body and the clothing by introducing a plastic material in a specific manner at the same time as the wire. Depending on the composition of the plastic material, this in turn led to unacceptable problems in the chemical treatment for producing the needle finish or in the final hardening of the nickel-diamond dispersion layer.
It has accordingly been hitherto impossible to carry out the deburring of clothing wires for opening rollers after the wires have been mounted on the roller bodies in such a way as to reduce, to a level acceptable to the consumer, the later occurrence of corrosion phenomena in the gap between the body and the wire and to apply an antiwear coating, for example a nickel-diamond dispersion layer to these opening rollers directly following deburring in a single multi-stage treatment process.